This invention relates to light emitting diode (LED) arrangements and, more particularly, to an LED arrangement providing improved contrast when lighted.
A light emitting diode, usually called an LED, has many advantages including, for example, low operating voltage, high response, small size, and long lifetime. Because of these advantages, LEDs are widely used as display lamps for various types of electronic devices and equipment.
As shown in FIG. 3, a conventional LED lamp includes a pair of lead terminals 21 and 22, an LED element 23 mounted at one end of the lead terminal 21, a metal wire 24 connecting the LED element 23 to one end of the second lead terminal 22, and a molded resin portion 25 encapsulating the end portions of the lead terminals 21 and 22 and the LED element 23 to seal them from the environment.
In one application of conventional LEDs, an array of LEDs is mounted on the control panel of an electric device to indicate the ON/OFF condition of switches in the electric device and to provide operating condition indicators on the control panel.
A display panel consisting of a matrix of conventional LED lamps, shown in FIGS. 4(A) and 4(B), has been widely used in highway message boards and in advertising boards. FIG. 4(A) is a front view of the conventional LED lamp and FIG. 4(B) is a side view of the same lamp. The matrix display panel includes a printed circuit board 27 containing a matrix of LED lamps 26, another printed circuit board 28 containing electronic parts for driving and controlling the matrix of LED lamps 26, and a housing 29 having a matrix of through-holes to receive the LED lamps 26. In operation, the LED lamps 26 thus arrayed are selectively turned on in response to control signals so as to display information consisting of characters, numerals, graphics, and the like.
When conventional LED lamp arrangements, such as control panels and display panels, are positioned in direct sunlight, the light is diffused in the molded portions of the unlighted LED lamps. As a result, the contrast between the lighted LED lamps and the unlighted LED lamps is poor so that it is difficult to discriminate the lighted LED lamps from the unlighted LED lamps and vice versa. Thus, visual recognition of the information to be provided by the LED lamps is difficult.
This recognition problem is particularly serious in matrix display panels which are installed in a bright outdoor location.